According to technological development and increased demand for mobile devices, demand for using secondary batteries as energy sources has rapidly increased. Among such secondary batteries, lithium secondary batteries having high energy density and operating potential, long cycle life, and low self-discharge rate are commercially available and widely used.
A lithium secondary battery has a structure in which an electrode assembly including a positive electrode, a negative electrode, and a porous separator disposed between the positive electrode and the negative electrode is impregnated in an electrolyte including a lithium salt, wherein the positive electrode and the negative electrode are each prepared by applying an active material on an electrode current collector. During a charging process, lithium ions of a positive active material are dissolved and intercalated into an active material layer of the negative electrode. During a discharge process, lithium ions in the active material layer are dissolved and intercalated into the positive active material. The electrolyte serves as a medium that transfers lithium ions between the negative electrode and the positive electrode.
The electrolyte generally includes an organic solvent and an electrolyte salt. For example, the electrolyte may be prepared by adding a lithium salt, such as LiPF6, LiBF4, or LiClO4, in a solvent mixture including high-dielectric cyclic carbonate, such as propylene carbonate or ethylene carbonate; and low-viscosity chain carbonate, such as diethyl carbonate, ethyl methyl carbonate, or dimethyl carbonate.
Since a lithium-containing halide salt such as a lithium-containing fluoride salt or a lithium-containing chloride salt that is generally used as an electrolyte salt sensitively reacts with water, the lithium-containing halide salt reacts with water during a preparation process of a battery or in a battery and thus produces HX (X=F, Cl, Br, I), which is a type of strong acid. Particularly, since a lithium salt, LiPF6, is unstable at a high temperature, its anion may be thermally decomposed, and thus an acidic material such as HF may be produced. When the acidic material exists in a battery, it unconditionally brings along undesired side reactions.
For example, a solid electrolyte interface (SEI) layer on a negative electrode surface may easily break due to high reactivity of the HX (X=F, Cl, Br, I), which induces continuous regeneration of the SEI layer, and thus an increase in interface resistance of a negative electrode may result due to an increase in a layer amount of the negative electrode. Also, due to positive electrode surface adsorption of lithium fluoride (LiF), which is a by-product of formation of hydrofluoric acid (HF), a positive electrode interface resistance may increase. Further, the HX may generate sudden oxidation in the battery which may dissolve or degrade positive and negative active material. Particularly, when a transition metal cation included in a lithium metal oxide, which is used as a positive active material, is dissolved, an additional negative electrode film may be formed as the cation electrodeposits on the negative electrode, which may thus result in a further increase in the negative electrode resistance.
The SEI layer is formed on a negative electrode surface when a carbonate-based polar non-aqueous solvent reacts with lithium ions in an electrolyte during an initial charging process of a lithium secondary battery. The SEI layer suppresses decomposition of the carbonate-based electrolyte on the negative electrode surface and thus serves as a protecting layer that stabilizes the battery. However, the SEI layer that is only formed by using an organic solvent and a lithium salt is somewhat insufficient to serve as a continuous protecting layer, and thus the SEI layer may be slowly destroyed by increased electrochemical energy and thermal energy when charging/discharging of the battery continues or when a fully-charged battery is stored at a high temperature. Then, a side reaction of decomposition caused by reaction between a negative active material surface exposed by the destruction of the SEI layer and an electrolyte solvent may continuously occur, and thus a resistance of the negative electrode may increase.
In addition to the cause described above, an interface resistance between an electrode and an electrolyte may increase due to various factors, and when the interface resistance increases, the performance of a battery such as a charge/discharge efficiency and life characteristics may generally deteriorate.
In order to resolve such problems, Patent Document 1 (JP1993-13088) discloses a method of improving a resistance of a lithium secondary battery by including vinylene carbonate (VC) in an electrolyte. However, a thin layer prepared by using the method still exhibits high resistance, and thus the method does not produce sufficient effects in terms of suppressing a resistance increase of a battery.
Also, Patent Document 2 (KR-2012-0011209) discloses an electrolyte solution for a lithium secondary battery including alkylene sulfate having a predetermined structure, an ammonium compound having a predetermined structure, and vinylene carbonate. According to this method, an SEI layer prepared by using the sulfate-based compound is advantageous as a resistance of the SEI layer is small and thus may improve low-temperature output characteristics of the battery, but the method cannot significantly improve life characteristics of the battery, and thus further improvement is needed.